in addition to; besides; as well as
Used to add another item or factor, often in formal or written contexts. Similar to に加え, but に加えて is slightly more common in spoken Japanese.
See also: に加え
In addition to Japanese, I can also speak English.
Besides the rain, the wind has also become stronger.
さらに is an adverb meaning 'furthermore' that can stand alone at the start of a sentence, while に加えて directly attaches after the noun it adds to.
そのうえ means 'moreover' and links two full clauses, whereas に加えて attaches to a noun phrase to add it to what precedes.
だけでなく means 'not only... but also' and is more colloquial, often used for contrast, while に加えて is neutral and simply adds an extra item.
From the verb 加える (くわえる, 'to add') in its te-form, used as a conjunction meaning 'adding to that'.